Simplify NDCube.to_nddata #2116

Job	Run time
sdist_verify	17s
core / Load tox environments	7s
cron	0s
core / py313 (ubuntu-latest)	1m 26s
test / Load tox environments	6s
docs / Load tox environments	8s
test / py314 (ubuntu-latest)	3m 39s
test / py312 (windows-latest)	2m 43s
test / asdf_schemas (ubuntu-latest)	1m 18s
test / py312 (macos-latest)	3m 5s
test / py311-oldestdeps (ubuntu-latest)	1m 25s
docs / build_docs (ubuntu-latest)	3m 27s
publish	0s
notify	0s
	17m 41s

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